If it's not an option in the options menu, you could probably do this with AutoHotKey, assuming Modern Warfare 3 PC plays nice. I don't own this version of the game, but perhaps someone who does can take a crack at a script.
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agent86Dec 27 '11 at 23:26

At least with the PC versions, there should be a "Aim Down the Sight" control and a "Toggle" version. Make sure you bind to the right one.
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Jeff MercadoDec 28 '11 at 0:11

1 Answer
1

i don't know the exact lables for these options as i only own the german version, but the first one is what you're looking for wile the second one seems to be the one you're using - just change your settings.